The present invention relates to an apparatus and method for regenerating an aged motor vehicle exhaust catalytic converter.
Three-way catalysts in an exhaust gas purification system degrade with use, owing to oxidation of the noble metals used in such catalysts, normally Pt, Pd and Rh. This oxidation is a result of the normally lean burn engine operating conditions, typically an air/fuel super-stoichiometric level of lambda .lambda.=1.01, which is set in order to achieve improved fuel economy, and a suitable balance between the purification efficiencies of the three catalysts for unburned hydrocarbons, CO and NOx.
Patent document EP 0 490 612 A1 discloses an engine control system in which timers measure lean and rich running periods which alternate on a time scale of seconds or less.
It has been proposed in patent document U.S. Pat. No. 4,729,220 that the rate of oxidation of Rh in a three-way catalyst system may be reduced by reducing the air/fuel ratio to stoichiometric .lambda.=1 or below, when the catalyst temperature exceeds 750.degree. C., depending also on engine load operating conditions. However, with this system it is necessary to use continuously temperature sensors and engine load sensors in order to set the air/fuel mixture, which is relatively complicated and expensive. Furthermore, in some conditions, for example in a hot ambient environment, or heavy engine load, the catalyst may be continuously above the 750.degree. C. limit meaning that the air/fuel mixture is set at stoichiometric or below for an extended period of time, which may result in non-optimal fuel consumption or catalyst efficiency.